August 4, 2011 | 10
Time to look at more of the frogs I encountered in Romania. In the previous article I discussed Western Palaearctic water frogs (the species of Pelophylax). Here in Europe, water frogs live alongside another group of ranid frogs – the brown frogs, the only frogs unambiguously and unquestionably associated with the generic name Rana. Well, I say ‘live alongside’, but brown frogs are decidedly more terrestrial than water frogs (as you might expect), often being found far from water (though often in damp microhabitats).
The best known and most widespread of them is the Common frog or Grass frog R. temporaria, the ‘original’ frog as recognised by Linnaeus. It’s the most cold tolerant of brown frogs, occurring throughout pretty much the whole of the UK and Scandinavia, even as far north as the Arctic Circle, and it’s the frog that I know best – indeed, it’s the only one that occurs where I live, and in the places where I grew up.
So imagine my excitement on being confronted, on several separate occasions, with a different brown frog while in Romania. Whoop. Yes, I was delighted and thrilled to see particularly long-legged, sharp-snouted brown frogs at several locations, including close to the Valea Cheii River and also deep in the woods at Râpa Rosie (an area with fantastic badlands, good for finding Cretaceous dinosaur fossils and also for lizards). These, very obviously, were Agile frogs R. dalmatina.
Like the Common frog and other brown frogs, the Agile frog has a dark ‘mask’ behind its eyes and surrounding its tympanum (eardrum). This feature is mentioned in every single text that discusses the identification of Old World frogs, but I wonder what (if any) advantage might be associated with its presence. Is it something to do with camouflage, heat absorption, or protecting the eyes from glare, or what? I don’t think anybody’s ever looked into this (though I’d love to be wrong). The tympanum in the Agile frog is particularly large compared to that of other brown frogs, and positioned closer to the eye than is usual.
Its especially long hindlimbs readily differentiate it from other brown frogs – the legs look long and slim even when folded up, but the easiest way to determine how long the legs are in a frog is to stretch out one of the animal’s legs forward and in parallel with its body to see where the heel is relative to the snout. I discussed this previously in connection with water frogs; evidently, you can use it on different genera. In most brown frogs the heel doesn’t extend beyond the snout, but in the Agile frog the legs are so long that the heel is well beyond the snout – as you can see in the photo here. Frogs are so flexible that this is unlikely to hurt them: it definitely doesn’t damage them in any way. Anyway, these particularly long legs of course make the Agile frog an especially good leaper (leaps of 2 m have been measured), and – I presume – explain its English name. The webbing between its toes is reduced compared to that of other Rana species, and much reduced compared to that of more aquatic ranids like the Pelophylax water frogs.
Early on in the year, Agile frogs lay clusters of 600-1000 eggs; their tadpoles are olive brown in colour and are unusual compared to those of other Rana species in reaching as much as 6 cm in total length – this makes a big Agile frog tadpole one of the largest tadpoles of the European region. SVL in an adult is normally about 6 cm, but individuals may reach 12 cm in the southern part of the species’s range (Laňka & Vít 1986).
Like the Common frog and also the Moor frog R. arvalis, the Agile frog is widespread across Europe, occurring from the Pyrenean region, north to Denmark and southern Sweden, and eastward to the shores of the Black Sea. It occurs around the coasts of the Mediterranean and is unusual among brown frogs in being found on Sicily and various of the Dalmatian and Ionian islands.
Toward and away from the Mediterranean: a history of Agile frogs
Phylogenetic work indicates that the Agile frog is closely related to the Italian agile frog R. latastei and Stream frog R. graeca (Barbadillo et al. 1997, Veith et al. 2003), both of which are endemic to the Mediterranean region. In turn, this ‘Agile frog clade’ is closest to an ‘Anatolian clade’ of brown frogs.
The divergence between the ‘Agile frog clade’ + ‘Anatolian clade’ and the ‘Rana temporaria species group’ apparently happened early on in brown frog evolution, with molecular clock data indicating that it occurred round about 4 Ma ago: that is, early in the Pliocene (Veith et al. 2003). Some authors therefore argue that speciation in European brown frogs only got underway after the Messinian salinity crisis (Mensi et al. 1992, Veith et al. 2003), when arid Late Miocene conditions would have made the Mediterranean region rather inhospitable for this cool-climate, moist habitat group of frogs (needless to say, the Messinian salinity crisis – the virtual drying up of the Mediterranean and associated aridification of the whole region – evidently had a major impact on the fortunes of many European tetrapod lineages). Climatic fluctuations during the Late Pliocene seem to have driven additional speciation events, and a more equable, more frog-friendly climate presumably allowed brown frogs to now colonise the Mediterranean region: fossils identified as R. dalmatina are known from the Late Pliocene, as are other members of the Agile frog + ‘Anatolian clade’ and ‘Rana temporaria species group’, so the fossil record seems consistent with this view.
A possible alternative to this ‘post-Messinian’ model is that the respective lineages already existed, and had done since the Miocene, but lived in Asia and only invaded Europe during the Pliocene. Indeed, most of the older fossil brown frogs – dating to the Late Miocene – are from Kazakhstan and Japan. An Early Miocene brown frog from Germany – identified as Rana cf. temporaria (Böhme 2001) [shown here] – suggests that European brown frog lineages were in Europe prior to the Messinian salinity crisis. However, the idea that extant brown frog lineages go back prior to the Pliocene is inconsistent with the molecular clock data (Mensi et al. 1992, Veith et al. 2003): those Miocene Asian brown frogs, and the Miocene fossil from Germany, haven’t been identified with certainty* and I think a very real possibility is that they’re actually outside the crown-group that includes the modern brown frog lineages being discussed here.
* Note that they’re all ‘cf’ records: ‘cf’ stands for ‘confer’. When used in a scientific name (e.g., Rana cf. temporaria) it means something like “this specimen is very similar to Rana temporaria but does not definitely belong to that species”.
Whether brown frogs moved into Europe after the Miocene, or whether they were already there, it seems that the Agile frog clade had its ‘centre of origin’ somewhere round about Italy or the Balkans (I say this based on the distribution of R. latastei and R. graeca). From this region, the Agile frog spread far and wide. While not occurring today in the western Iberian Peninsula or the north or far east, fossils show that it was more widespread during the Pleistocene than today, occurring in north-western Spain and further east in Ukraine than it does at present; it also seems to have persisted in England until Middle Saxon times (c. 600-950 AD) (Gleed-Owen 2000).
So far, the Agile frog remains common across most of its range; as is the case with other amphibian species that inhabit developed countries, the fragmentation of populations caused by roads and urban areas definitely has a deleterious impact and increases the likelihood of local extinction (Lesbarrères et al. 2006). Its persistence and success relies not only on healthy freshwater breeding pools, but also on the existence and maintenance of green corridors and healthy woodlands (Ficetola et al. 2006, Hartel et al. 2009).
In case it’s not obvious, I love this deep-history stuff, and it’s always frustrated me that this sort of information is so hard to find. Still, that’s why you’re here, right?
So, there we have it – more anuran stories from the beautiful country that is Romania. Is that it, done? Of course not; more later. Ahh, frogs.
For previous Tet Zoo articles on European anurans, see…
Refs – -
Barbadillo, L. J., García-Paris, M. & Sanchíz, B. 1997. Orígenes y relaciones evolutivas de la herpetofauna Ibérica. In: Pleguezuelos, J. M. (ed.) Distribución y Biogeografía de los Anﬁbos y Reptiles en Espana y Portugal. Monografías de Herpetología, vol. 3. Granada, pp. 47-100.
Böhme, M. 2001. The oldest representative of a brown frog (Ranidae) from the Early Miocene of Germany. Acta Palaeontologica Polonica 46, 119-124.
Ficetola, G. F., Valota, M. & De Bernardi, F. 2006. Temporal variability of spawning site selection in the frog Rana dalmatina: consequences for habitat management. Animal Biodiversity and Conservation, 29.2, 157-163.
Gleed-Owen, C. P. 2000. Subfossil records of Rana cf. lessonae, Rana arvalis and Rana cf. dalmatina from Middle Saxon (c. 600-950 AD) deposits in eastern England: evidence for native status. Amphibia-Reptilia 21, 57-65.
Hartel, T., Nemes, S., Cogălniceanu, D., Öllerer, K., Moga, C. I., Lesbarrères, D., Demeter, L. (2009). Pond and landscape determinants of Rana dalmatina population sizes in a Romanian rural landscape. Acta Oecologica, 35, 53-59
Laňka, V. & Vít, Z. 1986. Amphibians and Reptiles. Hamlyn, Twickenham.
Lesbarrères, D., Primmer, C. R., Lodé, T. & Merilä, J. 2006. The effects of 20 years of highway presence on the genetic structure of Rana dalmatina populations. Écoscience 13, 531-538.
Mensi, P., Lattes, A., Macario, B., Salvidio, S., Giacoma, C. & Baletto, E. 1992. Taxonomy and evolution of European brown frogs. Zoological Journal of the Linnean Society 104, 293-311.
Veith, M., Kosuch, J. & Vences, M. 2002. Climatic oscillations triggered post-Messinian speciation of Western Palearctic brown frogs (Amphibia, Ranidae). Molecular Phylogenetics and Evolution 26, 310-327.